Depends on your riding style and what you prefer. I like riding all mountail slight dh and love the feel of my hydros ability to stop and modulate. My wife hates my hydro brakes though because she maily rides on paved trails, she says their too sensitive. What kind of mechanical brakes do you currently have?

Avid Ball Bearing 7(BB7). By far the best mechanical disc brake out there. If your not sure on hydro brakes yet but want to upgrade, get the BB7's they will almost feel like a hydro and only cost about a third of the price. They are awesome brakes and they work very well